Permanent magnet for ultra-high vacuum and production process thereof

ABSTRACT

A permanent magnet useful in an ultra-high vacuum atmosphere, such as an undulator requiring the ultra-high vacuum atmosphere of less than 1×10 -9  Pa and which, has excellent magnetic characteristics, includes an R-Fe-B system permanent magnet having a Ti undercoat layer on a surface thereof, an external layer selected from TiN, AlN and Ti 1-x  Al x  N (x is 0.03 to 0.70), and an Al intermediate layer therebetween.

This application is a U.S. national phase of PCT/JP96/03717, filed Dec.20, 1996.

TECHNICAL FIELD

The present invention relates to a permanent magnet usable for anultra-high vacuum atmosphere, which possesses an excellent adherency ofa film layer coated thereon and good magnetic characteristics, and isapplicable to an undulator or a similar device commonly employed inultra-high vacuum atmosphere. More specifically the invention relates toa permanent magnet used in ultra-high vacuum and the production processof the permanent magnet; the permanent magnet having excellent magneticproperties.

BACKGROUND ART

A novel permanent magnet of R(referring at least one element ofrare-earth elements)-Fe-B system has been proposed (in Japan PatentApplication Laid-Open No. Sho 59-46008, and Japan Patent ApplicationLaid-Open No. Sho 59-89401), which is made of mainly rare-earth elementsrich in Nd or Pr and B and Fe (eventually, therefore, the R-Fe-B systemmagnet does not contain expensive elements such as Sm or Co) and hassuperior magnetic characteristics to those found in the conventionaltype of rare-earth cobalt magnets.

Although the Curie point of the aforementioned magnet alloy is reported,in general, to be in a temperature range from 300° C. to 370° C., theCurie point of the R-Fe-B system permanent magnet (Japan PatentApplication Laid-Open No. Sho 59-64733 and Japan Patent ApplicationLaid-Open No. Sho 59-132104) was improved to be higher than thatreported for the conventional type magnet by substituting a portion ofFe element by Co element. Moreover, in order to develop a new type ofpermanent magnet having an equivalent or higher Curie point and highermaximum energy product, (BH)max, than the aforementioned Co-containingR-Fe-B system permanent magnet and to improve the temperaturecharacteristics, particularly intrinsic coercive force, iHc, another newtype of Co-containing R-Fe-B system permanent magnet has been proposed(Japan Patent Application Laid-Open No. Sho 60-34005), in which theintrinsic coercive force iHc can be enhanced by maintaining an extremelyhigh value (BH)max of more than 25MGOe, by substituting a compositionalfraction of R (which mainly represents light-weight rare-earth elementssuch as Nd or Pr) in the Co-containing R-Fe-B system permanent magnetsby at least one element chosen form the element group comprising ofheavy-weight rare-earth elements including Dy or Th.

Conventionally, the ferrite magnet has been employed as a magnet used ina vacuum atmosphere with an order of 10⁻³ Pa. However, the ferritemagnet has relatively low magnetic properties, which are not high andsufficient enough to employ to the undulator.

There are several important items required for a satisfactory permanentmagnet used for ultra-high vacuum atmosphere of lower than 1×10⁻⁹ Pa;they include

(1) excellent magnetic characteristics,

(2) no generation nor exhaustion of absorbed or contaminated gas fromthe magnet surface, and

(3) maintaining the high level of vacuum of 1×10⁻⁹ Pa even after themagnet being installed to the relevant equipment.

Accordingly, the aforementioned R-Fe-B system magnets could have beenapplied to the undulator used in the ultra-high vacuum because of theirhigh magnetic properties. However, since the gas can easily be adsorbedon or absorbed in the R-Fe-B system magnets, the adsorbed or absorbedgas will be generated or exhausted from the magnet surface layer,causing a difficulty in maintaining the ultra-high vacuum of less than1×10⁻⁹ Pa. As a result, the conventional type of R-Fe-B system permanentmagnet cannot be used for the ultra-high vacuum atmosphere. In a casewhen the R-Fe-B system magnet, on which Ni-plating was surface-treatedfor an anti-corrosion purpose, is utilized in the ultra-high vacuum, themagnet cannot be placed inside the vacuum chamber, rather it isinstalled outside thereof in order to build the undulator or the similardevice. Accordingly, the equipment itself becomes much larger size andthe excellent magnetic properties found in the R-Fe-B system magnetcannot effectively be practiced.

Even with other types of R-Fe-B system magnets with which various metalsor polymeric resins are coated in order to improve the corrosionresistance of the R-Fe-B system magnets, the generation or exhaustion ofadsorbed/absorbed gas is unavoidable, resulting in that the usage ofsuch corrosion-resistant R-Fe-B system magnet is very limited for theultra-high vacuum atmosphere of, particularly, lower than 1×10⁻⁹ Pa.

It is, therefore, an object of the present invention to provide apermanent magnet having excellent magnetic characteristics which can beemployed for the undulator used in the ultra-high vacuum atmosphere.Furthermore, the permanent magnet according to the present invention hasa dense and strongly bonded surface coated layer thereon in order toprevent any gas generation or gas exhaustion out of the magnet surfacelayers; hence the presently invented magnet has a completely differentfeatures from the conventional type of corrosion-resistant R-Fe-B systemmagnet on which various coated film is applied for anti-corrosionpurpose.

DISCLOSURE OF INVENTION

In order to develop a permanent R-Fe-B system magnet having stable andexcellent magnetic characteristics and a dense and adherent coated filmonto the substrate so that a generation of adsorbed or absorbed gas canbe prevented, the present inventors have examined the forming of a thinTiN film on the surface of the permanent magnet. As a result, it wasfound that the following procedures were promising to achieve thepurpose. Namely, (1) the surface of the magnet body is cleaned by theion sputtering method. (2) A certain film thickness of Ti coated layeris formed on the cleaned surface of the magnet through a thin filmforming technique such as the ion plating method. (3) Nitrogen-diffusedlayer, TiN_(x) (x=0˜1), is formed through a thin film forming techniquesuch as the ion plating method using a mixed gas of Ar gas and N₂ gas insuch a manner that N concentration in the nitrogen-diffused layergradually increases toward the surface of the previously formed Ticoated layer. (4) Furthermore, a certain film thickness of TiN coatedlayer is formed through the ion reaction plating technique in N₂ gasatmosphere. It was found that the thus prepared permanent magnet can beused with the undulator in the ultra-high vacuum since the degree ofvacuum of less than 1×10⁻⁹ Pa was achieved after it was placed insidethe equipment. Moreover, after further investigation on the TiN thinfilm forming method on the surface of the permanent magnet, the presentinventors had found that the following procedures provided excellentresults on enhanced bond strengths between Al film and TiN film. Namely,the procedures are as follows. (1) The surface area of the permanentmagnet was cleaned by an ion sputtering technique. (2) A certainthickness of Ti coated film and Al coated film were subsequently formedby the thin film forming method such as the ion plating method. (3) Acertain thickness of TiN film was formed through the thin film formingmethod such as the ion reaction plating in N₂ gas. It was found that theTiN film exhibited an excellent bond strength to the Ti under coatedfilm. (4) While forming the TiN film coated on the Al film, a complexfilm having a formula Ti₁₋α Al.sub.α N.sub.β (where o<α<1, and 0<β<1)was formed. The composition and the film thickness of Ti₁₋α Al.sub.αN.sub.β were varied depending upon the magnet substrate temperature, thebias voltage, and the film growth rate. Accordingly, compositionalfraction of Ti and N were continuously increasing toward the TiNinterface, so that the excellent bond strength between Al coated filmand TiN coated film was achieved.

Furthermore, the present inventors have discovered that, while AlNcoated layer was formed on Al coated layer after Ti coated layer and Alcoated layer were subsequently formed onto the permanent magnet surface,a complex film composed of Al and N having a formula AlN_(x) was formedat the interface. The composition and film thickness of the complexAlN_(x) were varied depending upon the temperature of the magnetsubstrate, the bias voltage, and the film growth rate. It was also foundthat the N concentration increased gradually toward to the AlNinterfacial area, leading to that the adherency between Al coated layerand the AlN film was remarkably enhanced.

Moreover, the present inventors have investigated the method forproducing another type of complex compound Ti_(1-x) Al_(x) N onto thesurface layer of the permanent magnet. As a result, a certain filmthickness of Ti_(1-x) Al_(x) N can be formed through the thin filmforming method such as the ion reaction plating technique operated inthe Nitrogen-containing gas, after Ti coated layer and Al coated layerwere subsequently formed. Namely, when Ti_(1-x) Al_(x) N film was formedonto the Al coated layer, it was found that an intermediate complexcompound, Ti₁₋α Al.sub.α N.sub.β (where 0<α<1, and 0<β<1), was formed atthe interfacial area. The composition and the film thickness of theformed Ti₁₋α Al.sub.α N.sub.β varied depending upon the temperature ofthe magnet substrate, the bias voltage, the film growth rate, and thecomposition of Ti_(1-x) Al_(x) N. Compositional fraction of Ti and Nappeared to gradually increase toward to the interface with Ti_(1-x)Al_(x) N layer, resulting in a remarkably improved bond strength betweenAl coated layer and the Ti_(1-x) Al_(x) N layer.

The above and many other objectives, features and advantages of thepresent invention will be fully understood from the ensuing detaileddescription of the examples of the invention, which description shouldbe read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a ultra-high vacuum equipment with which the pressure ofvacuum was measured.

FIGS. 2 through 5 show the progressive changes in degree of vacuum fordifferently surface-treated magnets, indicating the time required toreach the pressure of vacuum.

BEST MODE FOR CARRYING OUT THE INVENTION

An method example for producing a permanent magnet used in theultra-high vacuum atmosphere will be described in the followingsequences, in which said permanent magnet is further characterized byproviding TiN layer being coated onto Ti coated layer, which waspreviously provided on the surface of the R-Fe-B system permanentmagnet, through the nitrogen-diffused layer (having a composition ofTiN_(x)) in which N concentration increased gradually.

(1) In the arc ion plating equipment, after the vacuum chamber wasevacuated below the pressure of vacuum of 1×10⁻³ Pa, the surface area ofR-Fe-B system permanent magnet was cleaned by the surface sputter of Arion in Ar gas pressure of 5 Pa and at the voltage of -600V.

(2) In the next step, Ti element as a target material was evaporated bythe arc ion plating under an Ar gas pressure of 0.2 Pa, and the biasvoltage of -80V to produce a Ti coated layer with a film thickness from0.1 μm to 5.0 μm.

(3) Subsequently, in order to form a certain thickness of thenitrogen-diffused layer with a composition of TiN_(x) on Ti coatedsubstrate layer, while Ti was kept to be evaporated, the magnetsubstrate temperature was also kept at 400° C. After introducing a mixedgas of Ar gas and N₂ gas under a gas pressure of 1 Pa, the bias voltageof -120V, and arc current of 80A, a nitrogen-diffused layer was formedin such a manner that N₂ concentration gradient was continuouslyincreasing toward the TiN coated layer by increasing N₂ amount.

(4) In the final step, by the arc ion plating under N₂ gas pressure of1.5 Pa, a certain thickness of TiN coated layer was formed on thenitrogen-diffused layer. According to the present invention, althoughany prior art methods for forming thin films including the ion platingmethod or the evaporation method can be employed in order to form the Ticoated layer and nitrogen-diffused layer on the surface of R-Fe-B-systempermanent magnet, it is preferable to utilize either ion plating methodor ion reaction plating method from standpoints of the density,uniformity and growth rate of the formed film. It is preferable to setthe heating temperature of the magnet substrate in a temperature rangefrom 200° C. to 500° C. during the reaction film forming process. If itis lower than 200° C., a sufficient bond strength was not obtainedbetween the reaction film and the magnet substrate; while if it exceeds500° C., undesired cracking will take place in the films during thecooling stage, causing the peeling off from the magnet substratesurface; so that it is better to set the magnet substrate temperatureranging between 200° C. and 500° C.

In this invention, the main reason for defining the film thickness in arange from 0.1 μm to 3.0 μm for Ti film coated on the magnet surface wasdue to the facts that (1) if it is less than 0.1 μm, it is not thickenough to maintain the sufficient bond strength, and (2) if it exceeds3.0 μm, although no adverse effect is recognized with respect to thebond strength, it will cause the cost-up and is not practical.

Similarly, main reasons for controlling the film thickness ofnitrogen-diffused layer in a range from 0.05 μm to 2.0 μm being formedon Ti coated layer were due to the facts that (1) if it is less than0.05 μm, the thickness of the diffusion layer is not thick enough, andon the other hand, (2) if it exceeds 2.0 μm, although no adverse effecton bond strength, it will cause raise in the production cost and henceis not practical.

It is preferable, in this invention, for the nitrogen-diffused layerformed on the Ti coated layer to have a gradually increased N₂concentration toward the TiN coated layer.

Moreover, the main reason for controlling the film thickness of TiNcoated layer in a range from 0.5 μm to 10 μm were due to the facts that(1) if it is less than 0.5 μm, sufficient corrosion resistance as wellas wear resistance being characterized with TiN cannot be realized, onthe other hand, (2) if it exceeds 10 μm, although no problems withrespect to its effectiveness, it will cause the raise in the productioncost.

In the following, an example procedure for producing the permanentmagnet will be described, in which said magnet is characterized byforming TiN coated layer through the Al coated layer which was formed onthe Ti coated film, after the Ti film was formed on surface of theR-Fe-B system permanent magnet.

(1) In the arc ion plating equipment, after evacuating the vacuumchamber less than the target degree of vacuum of 1×10⁻³ Pa, the surfacearea of the R-Fe-B system permanent magnet was cleaned by the surfacesputtering Ar ion under Ar gas pressure of 5 Pa and voltage of -600V.

(2) After evaporating the Ti element as a target material under Ar gaspressure of 0.1 Pa and the bias voltage of -50V, Ti coated film with afilm thickness ranging from m to 3.01 m was formed on the magnet surfacethrough the arc ion plating method.

(3) After evaporating the target Al under the Ar gas pressure of 0.Paand the bias voltage of -50V, Al coated film with a film thicknessranging from 1 μm to 5 μm was formed on the Ti coated layer through thearc ion plating method.

(4) Using Ti as a target material, while keeping the magnet substratetemperature at 250° C., a certain film thickness of TiN was formed onthe Al coated layer under N₂ gas pressure of 1 Pa, the bias voltage of-100V, and arc current of 100A.

According to the present invention, the main reason for controlling thefilm thickness of Al coated layer in a range of 0.1 μm and 5.0 μm aredue to the facts that (1) if it is less than 0.1 μm, Al element is hardto deposited uniformly onto the Ti coated layer and the effectivefunction as an intermediate layer is not achieved, on the other hand,(2) if it exceeds 5.0 μm, although the function as an intermediate layeris not deteriorated, it will cause the raise in production cost.

The main reasons for setting the film thickness of TiN in a range from0.5 μm to 10 μm are due to the facts that (1) if it is less than 0.5 μm,the sufficient corrosion resistance and wear resistance cannot beachieved, on the other hand, (2) if it exceeds 10 μm, it will cause araise in the production cost although it does not affect any adverseinfluence on its functionality.

An example procedure for producing the permanent magnet will bedescribed in the followings, which said magnet is characterized byproviding Ti coated layer, and AlN coated layer through the Al coatedlayer on the Ti coated layer on the R-Fe-B system permanent magnet.

(1) In the arc ion plating equipment, after the vacuum chamber isevacuated at less than the target degree of vacuum of 1×10⁻³ Pa, thesurface of the R-Fe-B system permanent magnet was cleaned by surfacesputtering Ar ion under the Ar gas pressure of 10 Pa and the voltage of-500V.

(2) Ti as a target material was evaporated under the Ar gas pressure of0.1 Pa and the bias voltage of -80V in order to form the Ti coated layerwith a film thickness ranging from 0.1 μμm to 3.0 μm on the magnetsubstrate through the arc ion plating method.

(3) Similarly, Al was evaporated under the Ar gas pressure of 0.1 Pa andthe bias voltage of -50V in order to form the Al coated layer with afilm thickness-ranging from 0.1 μm to 5.0 μm on Ti coated layer by thearc ion plating method.

(4) Using Al as a target material and keeping the magnet substratetemperature at 250° C., AlN film was formed with a certain filmthickness onto the Al coated layer under the N₂ gas pressure of 1 Pa andthe bias voltage of -100V.

The main reasons for the controlling the film thickness of the Al coatedlayer from 0.1 μm to 5 μm are due to the facts that (1) if it is lessthan 0.1 μm, Al element is hardly deposited uniformly onto the Ti coatedlayer and does not perform the sufficient function as the intermediatelayer, on the other hand, (2) if it exceeds 5 μm, it will increase theproduction cost although it does not show any adverse effect.

Moreover, the main reasons for controlling the AlN film thickness in arange from 0.5 μm to 10 μm are due to the facts that (1) if it is lessthan 0.5 μm, sufficient corrosion resistance as well as wear resistancecannot be achieved, on the other hand, (2) if it exceeds 10 μm, althoughit does not show any adverse effects on the efficiency, it will increasethe production cost.

In the followings, an example procedure for producing the permanentmagnet will be described, in which said permanent magnet ischaracterized by providing Ti_(1-x) Al_(x) N (where 0.03<x<0.70) coatedlayer through the Al coated layer being previously formed on the Ticoated layer, after forming Ti coated layer onto the surface of R-Fe-Bsystem permanent magnet.

(1) In the arc ion plating equipment, the vacuum chamber was evacuatedbelow the pressure of vacuum of 1×10⁻³ Pa, the surface area of theR-Fe-B system permanent magnet was cleaned by surface sputtering Ar ionunder Ar gas pressure of 10 Pa and the voltage of -500V.

(2) Ti as a target material was evaporated under Ar gas pressure of 0.1Pa and the bias voltage of -80V in order to form the Ti coated layerwith a film thickness ranging from 0.1 μm to 3.0 μm onto the magnetsubstrate by the arc ion plating method.

(3) Al as the next target material was evaporated under the Ar gaspressure of 0.1 Pa and the bias voltage of -50V in order to form the Alcoated layer with film thickness ranging from 0.1 μm to 5 μm onto Ticoated layer by the arc ion plating technique.

(4) Subsequently, using an alloy Ti_(1-x) Al_(x) (where 0.03<x<0.80) asa target material and keeping the magnet substrate temperature at 250°C., a certain film thickness of Ti_(1-x) Al_(x) N coated film was formedonto the Al coated layer under the N₂ gas pressure of 3 Pa and the biasvoltage of -120V.

According to the present invention, the main reasons for defining thethickness of Al coated layer onto the Ti coated layer in a range from0.1 μm to 5 μm are due to the facts that (1) if it is less than 0.1 m Alis hardly deposited uniformly on Ti coated layer and does not functionas an intermediate layer, and (2) if it exceeds 5 μm, it will cause araise in the production cost, although it does not affect any adverseeffect on the efficient functionality.

Moreover, the main reasons for defining the film thickness of Ti_(1-x)Al_(x) N (where 0.03<x<0.70) coated layer in a range from 0.5 μm to 10μm are due to the facts that (1) if it is less than 0.5 μm, sufficientcorrosion resistance and wear resistance cannot be achieved, and that(2) if it exceeds 10 μm, although no problem with respect to theefficiency, it will cause the raise in production cost. Furthermore, inthe composition Ti_(1-x) Al_(x) N, if x is less than 0.03, thesufficient properties of the corrosion resistance as well as wearresistance cannot be obtained; while if it exceeds 0.70, no remarkableimprovement in properties were recognized and it is hard to obtain theuniformly distributed composition.

The rare-earth element, R, used in the permanent magnet of the presentinvention has a composition ranging from 10 atomic % to 30 atomic %. Itis preferable to choose at least one element from a element groupcomprising of Nd, Pr, Dy, Ho, and Tb, and/or at least one element from aelement group consisted of La, Ce, Sm, Gd, Er, Eu, Tm, Yb, Lu, and Y.Normally it would be good enough if one element R was selected. However,it would be more practical and efficient if a mixture of more than twoelements (such as mishmetal or didymium) were preferably chosen.Furthermore, it is not necessary to select the pure grade rare-earthelement, rather any element(s) containing unavoidable impurity orimpurities can be selected.

The R element is an essential element for the permanent magnet. If it iscontained less than 10 atomic %, since the crystalline structure of theR element is a cubic structure, which is identical to that of α-Fe(ferrite), then excellent magnetic properties, particularly highintrinsic coercive force cannot be obtained. On the other hand, if itexceeds 30 atomic %, a R-rich non-magnetic phase will become to be adominant phase, causing a reduction in the residual flux density, Br, sothat the permanent magnet with excellent magnetic characteristics cannotbe produced. Accordingly, it is preferable to control the R contents ina range from 10 atomic % to 30 atomic %.

Boron, B, is also an essential element for the permanent magnet. If itis contained less than 2 atomic %, the rhombohedral structure willbecome to be a parent phase, resulting in that high intrinsic coerciveforce, iHc, cannot be expected. On the other hand, if it exceeds 28atomic %, the B-rich non-magnetic phase will be a dominant phase,resulting in a reduction in the residual flux density, Br, so that thepermanent magnet with excellent magnetic properties cannot be produced.Accordingly, it is preferable to control the B contents in a range from2 atomic % to 28 atomic %.

It is obvious that Fe element is the essential element for the permanentmagnet. If it is contained less than 65 atomic %, the residual fluxdensity, Br, will be reduced; on the other hand, if it exceeds 80 atomic%, high value of intrinsic coercive force, iHc, cannot be expected.Hence, it is preferable to control Fe contents in a range between 65atomic % and 80 atomic %. Although a substitution of a fraction of Fewith Co will improve the temperature characteristics withoutdeteriorating other magnetic properties; if Co is replaced to more than20% of Fe element, the magnetic property will be adversely influenced.If amount of replacing Co is within a range of 5 atomic % to 15 atomic %of the total amount of Fe and Co elements, the residual flux density,Br, will increase, compared to the magnet without any replaced Coelement, so that a range between 5 atomic % and 15 atomic % ispreferable in order to obtain the high residual flux density.

Unavoidable impurity (or impurities) will be allowed to theaforementioned three essential elements, R, B, and Fe. For example, Aportion of B element can be replaced by at least one element from theelement group comprising of C (less than 4.0 weight %), P (less than 2.0wt %), S (less than 2.0 wt %) and Cu (less than 2.0 wt %) or anyelements if the total percentage is less than 2.0 wt %. It is possibleto improve the productivity and the cost-down for fabricating thepermanent magnets if the above mentioned substitution is conducted.

Furthermore, at least any one of element selected from the element groupconsisted of Al, Ti, V, Cr, Mn, Bi, Nb, Ta, Mo, W, Sb, Ge, Sn, Zr, Ni,Si, Zn, and Hf can be added to the R-Fe-B system permanent magnet inorder to improve the intrinsic coercive force, the rectangularity ofdemagnetization curve, a productivity, and cost-performance. The upperlimit of the addition should be carefully selected, since the residualflux density Br is required to show at least more than 9 kG in order tohave the (BH)max being higher than 20MGOe.

Moreover, the permanent magnet of the present invention is characterizedby the fact that a parent phase of the magnet is a tetragonalcrystalline structure having an average grain size ranging from 1 μm to80 μm, and that the magnet contains 1% to 50% (in the volumetric ratio)of non-magnetic phase (excluding oxide phase(s)).

The permanent magnet, according to the present invention, shows thefollowing magnetic characteristics; namely, the intrinsic coerciveforce, iHc≧1 kOe, the residual flux density, Br>4 kG, the maximum energyproduct, (BH)max≧10 MGOe, while the maximum value can reach more than 25MGOe.

Example 1--1

A prior art of cast ingot was pulverized, followed by press-forming,sintering and heat-treating the product to prepare a sample magnethaving a composition of 15 Nd-1Dy-77Fe-7B with a dimension of 12 mm indiameter and 2 mm in thickness. The sample magnet was placed in thevacuum chamber to evacuate less than 1×10⁻³ Pa. After the surface of thesample magnet was cleaned under the surface Ar ion sputter method underAr gas pressure of 5 Pa and the voltage of -600V for 20 minutes, Tielement as a target element was then plated with a film thickness of 0.5μm on the surface of the sample magnet under following conditions; Argas pressure: 0.2 Pa, bias voltage: -80V, arc current: 120A, andtemperature of the magnet substrate: 380° C.

After the magnet substrate was heated again at 380° C. and a mixed gas(Ar:N₂ =9:1) with a pressure of 1 Pa was introduced. While the mixedratio of Ar and N₂ gas was continuously changed from the initial ratioof 9:1 to 7:3→5:5→3:7→0:10, a nitrogen-diffused layer (with acomposition TiN_(x)) with a film thickness of 0.2 μm was formed on theTi coated layer under the bias voltage of -120V and arc current of 80Afor 30 minutes.

Furthermore, the TiN coated layer with a film thickness of 5 μm wasformed on the aforementioned nitrogen-diffused layer through the ionplating technique under the following conditions; N₂ gas pressure: 1.5Pa, bias voltage: -100V, arc current: 120A.

After the chamber cooling, magnetic properties of the thus preparedpermanent magnet having TiN layer were measured. The obtained resultsare listed in Table 1. The time required for the reaching the targetdegree of vacuum, using the above prepared sample magnet, was alsomeasured by the ultra-high vacuum equipment, as seen in FIG. 1. FIG. 2shows the results on the progressive changes in the degree of vacuum.

In the ultra-high vacuum equipment as seen in FIG. 1, there are anultra-high vacuum chamber 1, a main body of cylindrical tube 2, in whicha Ti getter pump 4, an ion pump 5, BA gage 6 and an extractor gage 7 areplaced. A sample chamber 3 is provided at one end portion of the mainbody 2.

Without placing the sample magnet 8 into the vacuum chamber 3, thechamber was baked at a temperature of 150° C.˜200° C. for 48 hours whileevacuating the chamber with operating the Ti getter pump 4 and the ionpump 5. After the temperature inside of the main body 2 was cooled downlower than 70° C., the final reachable target degree of vacuum wasmeasured by operating the BA gage 6 and the extractor gage 7. It wasrecorded that the finally reached target degree of vacuum was 7×10⁻¹⁰Pa, as seen with a line "a" in FIG. 2.

Sixty (60) pieces of sample magnets 8 with dimension of 8 mm high×8 mmwide×50 mm long were placed inside the sample chamber 3. After bakingthe chamber at a temperature of 150° C.-200° C. for 48 hours byoperating the Ti getter pump 4 and the ion pump 5. After the temperatureof the main body 2 was cooled down below 70° C., the degree of vacuumwas progressively measured by operating the BA gage 6 and the extractorgage 7. The time elapsed until the final target degree of vacuum wasshown with the curve "b" in FIG. 2, where ◯ marks represent data pointmeasured by the BA gage and □ marks indicate data points obtained withthe extractor gage.

Comparison 1-1

Magnetic properties of the sample magnet having an identical compositionas the previous Example 1--1 are also listed in Table 1. After samplemagnets with identical dimensions and quantity as the Example 1--1 werecleaned under the same conditions conducted for the Example 1--1, thetarget degree of vacuum was measured with the ultra-high vacuum chamberof FIG. 1 under the same conditions performed for the Example 1--1. Theresult is shown with the curve "c" in FIG. 2.

                  TABLE 1                                                         ______________________________________                                                      magnetic properties                                                           Br(kG)                                                                              iHc(kOe) (BH)max(MGOe)                                    ______________________________________                                        Example 1-1                                                                           this invension                                                                            11.6    16.8   32.8                                       Comparison                                                                            un-treated magnet                                                                         11.7    16.6   33.2                                       1-1                                                                           Comparison                                                                            Ni-plated magnet                                                                          11.5    16.4   32.6                                       1-2                                                                           ______________________________________                                    

Comparison 1-2

Same number of sample magnets with identical dimensions and compositionsas the Example 1--1 were used. After the surface area of the samplemagnets were cleaned under the same conditions done for the Example 1-1,Ni film with a thickness of 20 μm was formed by a conventional platingmethod. The magnetic properties of the Ni-plated magnets were evaluatedand listed in Table 1. The surface area of the Ni-plated magnets werecleaned, followed by measurement on the pressure of vacuum using theultra-high vacuum chamber of FIG. 1 under the same conditions performedfor the Example 1--1. The data is shown with the curve "d" in FIG. 2.

The R-Fe-B system permanent magnet, according to the present invention,being provided with the TiN layer onto the Ti coated layer through thenitrogen-diffused layer (with a composition of TiN_(x)) withcontinuously increased N concentration has demonstrated clearly that nogas was generated out of the magnet surface, so that the vacuum of1×10⁻⁹ Pa was achieved. On the other hand, with un-treated magnet orNi-plated magnet, it was found that the gas generation cannot beprevented. So that the target degree of vacuum was not achieved.

Example 2-1

The cast ingot of the prior art was pulverized, followed bypress-forming, sintering and heat-treating to produce a sample magnet of16Nd-1Dy-76Fe-7 B with dimensions of 12 mm in diameter and 2 mm inthickness. The measured magnetic properties are listed in Table 2.

The vacuum chamber was evacuated under the level of 1×10⁻³ Pa. Thesurface area of the sample magnet was cleaned by the surface Ar ionsputter under the Ar gas pressure of 10 Pa and the voltage of -500V for20 minutes. Keeping the Ar gas pressure at 0.1 Pa, the bias voltage at-80V, arc current at 100 A and the temperature of the magnet substrateat 280° C., the Ti coated layer with a film thickness of 1 μm was formedonto the magnet surface by using Ti as a target material through the arcion plating technique.

Furthermore, under the conditions such as Ar gas pressure of 0.1 Pa,bias voltage of -50V, arc current of 50A, and the magnet substratetemperature at 250° C., the Al coated layer with a film thickness of 2μm was formed onto the Ti coated layer by using metallic Al as a targetmaterial through the arc ion plating method.

Under the magnet substrate temperature of 350° C., bias voltage of-100V, arc current of 100A, N₂ gas pressure of 1 PA, the TiN coatedlayer with a film thickness of 2 μm was formed onto the Al coated layerthrough the arc ion plating by using metallic Ti as a target material.

After the chamber cooling, the magnetic properties of the permanentmagnet with TiN coated film were examined. Results are shown in Table 2.The pressure of vacuum of the permanent magnet was measured with theultra-high vacuum equipment, as seen in FIG. 1. The obtained results areseen in FIG. 3.

                  TABLE 2                                                         ______________________________________                                                      magnetic properties                                                           Br(kG)                                                                              iHc(kOe) (BH)max(MGOe)                                    ______________________________________                                        Example 2-1                                                                           this invension                                                                            11.2    15.9   30.1                                       Comparison                                                                            un-treated magnet                                                                         11.7    15.9   30.1                                       2-1                                                                           Comparison                                                                            Ni-plated magnet                                                                          11.1    15.9   30.1                                       2-2                                                                           ______________________________________                                    

The measuring procedures were exactly same as those performed for theExample 1--1. The final reachable degree of vacuum of the used equipmentwas 7×10⁻¹⁰ Pa, as indicated with the line "a" in FIG. 3. After sixty(60) pieces of sample magnets 8 with dimensions of 8 mm high×8 mmwide×50 mm long were placed inside the sample chamber 3, the timerequired until the final degree of vacuum elapsed was monitored, as seenin curve "e" in FIG. 3. Data points marked by 0 symbols representresults obtained by the BA gage; while □ marks indicate data pointsobtained with the extractor gage.

Comparison 2-1

The magnetic characteristics of the sample magnet having identicalcomposition as the Example 2-1, but without Ti film, Al coated layer,and TiN film layer are listed in Table 2. Identical number of samplemagnets with identical dimensions as the Example 2-1 were cleaned underthe same conditions conducted for the Example 2-1. The final reachabletarget degree of vacuum was measured under the same conditions done forthe Example 2-1 by using the ultra-high vacuum equipment of FIG. 1.Results are shown with the curve "f" in FIG. 3.

Comparison 2-2

After the surface area of identical number, identical composition andsize to those used for the Example 2-1 was cleaned under the sameconditions employed for the Example 2-1, the Ni film with a filmthickness of 20 μm was plated through the conventional platingtechnique. The magnetic properties of the thus prepared Ni-plated magnetwere evaluated and results are listed in Table 2. Subsequently, afterthe Ni-plated surface was cleaned, the final reachable degree of vacuumwas measured under the same conditions done for the Example 2-1 by usingthe ultra-high vacuum equipment of FIG. 1. The results are shown withthe curve "g" in FIG. 3.

It was found that the R-Fe-B system permanent magnet, according to thepresent invention, being provided with TiN coated layer through the Alcoated layer which was previously formed on the Ti coated layer hasdemonstrated no gas generation out of the magnet surfaces and asatisfactory capability of reaching the final pressure of vacuum of1×10⁻⁹ Pa. On the other hand, the magnet without any treatment or thosewith Ni-plated layers thereon showed the gas generation, so that thefinal reachable target degree of vacuum was not achieved.

Example 3-1

The cast ingot of the prior art was pulverized, followed bypress-forming, sintering and heat-treating in order to produce thesample magnet having a composition of 16Nd-1Dy-75Fe-8B and dimensions of12 mm in diameter and 2 mm in thickness. After the sample magnet wasplaced inside the vacuum chamber, it was evacuated below the degree ofvacuum of 1×10⁻³ Pa. After the surface area of the magnet was cleaned bythe surface Ar ion sputter method under the conditions of Ar gaspressure of 5 Pa, voltage of -600V for 20 minutes, the Ti coated layerwith a film thickness of 1 μm was formed on the magnet surface throughthe arc ion plating method using metallic Ti as a target material underthe following conditions; namely, Ar gas pressure: 0.2 Pa, bias voltage:-80V, the magnet substrate temperature: 250° C.

Subsequently, keeping the Ar gas pressure at 0.1 Pa, bias voltage at-50V and the magnet substrate temperature at 250° C., the Al coatedlayer with a film thickness of 2 μm was formed onto the Ti coated layerthrough the arc ion plating technique using metallic Al as a targetmaterial. In the next stage, the AlN coated layer with a film thicknessof 2 μm was formed on Al coated layer by the arc ion plating methodusing metallic Ti as a target material under the conditions of magnetsubstrate temperature of 350° C., the bias voltage of -100V, and N₂ gaspressure of 1 Pa.

After the chamber cooling, the magnetic properties of the thus preparedmagnet was measured. The results are listed in Table 3. The reachablepressure of vacuum was evaluated using the ultra-high vacuum equipmentof FIG. 1. The obtained results are shown in FIG. 4.

The measuring procedures for the Example 3-1 were exactly same as thosedone for the Example 1-1. It was found that the final reachable degreeof vacuum was 7×10⁻¹⁰ Pa, as seen with the line "a" in FIG. 4. Aftersixty pieces of sample magnets 8 with dimensions of 8 mm high×8 mmwide×50 mm long were placed inside the sample chamber 3, the timerequired for the final reachable degree of vacuum was monitored, as seenthe curve "h" in FIG. 4, where ◯ marks represent data points obtained bythe BA gage and L marks indicate data points measured by the extractorgage.

Comparison 3-1

The magnetic properties of sample magnet having identical composition asthose used for the Example 3-1 but without any external films of Ticoated layer, Al coated layer, and AlN coated layer are also listed inTable 3. After the surface area of identical numbers of sample magnetswith identical dimensions to those used in the Example 3-1 was cleanedunder the same procedures conducted for the Example 1-1, the finalreachable pressure of vacuum was measured under the same conditionsperformed for the Example 3-1 using the ultra-high vacuum equipment ofFIG. 1. The result is shown with the curve "i" in FIG. 4.

                  TABLE 3                                                         ______________________________________                                                      magnetic properties                                                           Br(kG)                                                                              iHc(kOe) (BH)max(MGOe)                                    ______________________________________                                        Example 3-1                                                                           this invension                                                                            11.3    16.0   30.1                                       Comparison                                                                            un-treated magnet                                                                         11.3    16.0   30.1                                       3-1                                                                           Comparison                                                                            Ni-plated magnet                                                                          11.2    16.0   30.0                                       3-2                                                                           ______________________________________                                    

Comparison 3-2

After surface area of same numbers of sample magnets with identicalcomposition and dimensions to those used for the Example 3-1 was cleanedunder the same procedures done for the Example 3-1, Ni-plated film witha film thickness of 20 μm was formed through the conventional platingmethod. The magnetic properties of the Ni-plated sample magnet are alsolisted in Table 3. Furthermore, after the surface layer of the Ni-platedmagnet was cleaned, the final reachable pressure of vacuum was measuredunder the same conditions as those conducted for the Example 1-1 usingthe ultra-high vacuum equipment of FIG. 1. The result is shown with thecurve "j" in FIG. 4.

The R-Fe-B system permanent magnet, according to the present invention,being provided with TiN coated film and subsequently formed AlN filmcoated on Al film which was previously coated on said Ti film hasclearly demonstrated that no gas was generated from the magnet surface,so that the degree of vacuum of 1×10⁻⁹ Pa or less can be achieved.However, with sample magnets with either untreated condition orNi-plated film, gas generation was noticed, so that the target degree ofvacuum cannot be achieved.

Example 4-1

The cast ingot of the prior art was pulverized, followed bypress-forming, sintering and heat-treating in order to produce thesample magnet with a composition of 16Nd-76Fe-8B with dimensions of 12mm in diameter and 2 mm in thickness. After the magnet was placed insidethe vacuum chamber, the chamber was evacuated below the level of 1×10⁻³Pa. After the surface area of the magnet was cleaned under the surfacesputter method under the conditions of the Ar gas pressure of 5 Pa andvoltage of -600V for 20 minutes, the Ti coated layer with a filmthickness of 1 μm was formed by the arc ion plating method usingmetallic Ti as a target material under the conditions of Ar gas pressureof 0.2 Pa, bias voltage of -80V, and the magnet substrate temperature at250° C.

Subsequently, the Al coated layer with a film thickness of 2 μm wasformed onto the Ti coated layer through the arc ion plating technique byusing metallic Al as a target material under the conditions of the Argas pressure of 0.1 Pa, the bias voltage of -50V and the magnetsubstrate temperature of 250° C.

Keeping the magnet substrate temperature at 320° C., bias voltage of-120V and the N₂ gas pressure of 3 Pa, the Ti_(1-x) Al_(x) N film with afilm thickness of 3 μm was formed onto the Al coated layer through thearc ion plating technique by using an alloy Ti₀.4 Al₀.6 as a targetmaterial. It was found that the composition of the obtained complexcompound was Ti₀.45 Al₀.55 N. After the chamber cooling, the magneticproperties of the magnet was evaluated. Results are listed in Table 4.The final reachable pressure of vacuum was examined using ultra-highvacuum equipment of FIG. 1. The obtained results are shown in FIG. 5.

The same procedures as for the Example 1-1 were conducted for measuringthe final reachable degree of vacuum. It was found that the finallyreached degree of vacuum was 7×10⁻¹⁰ Pa, as seen with the line "a" inFIG. 5. After sixty pieces of sample magnets 8 with dimensions of 8 mhigh×8 mm wide×50 mm long were placed into the sample chamber 3, thetime required in order to reach the final pressure of vacuum wascontinuously monitored. The curve "k" in FIG. 5 shows the results,whereby 0 marks indicate data point obtained by the BA gage; while datapoint marked with ◯ symbols represent those obtained by the extractorgage.

Comparison 4-1

The magnetic properties of the sample magnet having the identicalcomposition as the Example 4-1, but without any coated films of Ti, Aland Ti_(1-x) Al_(x) N layers, are listed in Table 4. Similarly as donefor the Example 4-1, the surface area of the sample magnets werecleaned, and the finally reachable degree of vacuum was monitored in theultra-high vacuum equipment under the same conditions conducted for theExample 4-1. The line "1" in FIG. 5 shows the results.

Comparison 4-2

Sample magnets having identical composition, dimensions and quality asthose for the Example 4-1 were subjected to the surface cleaning underthe same conditions performed for the Example 4-1. Using theconventional plating method, the Ni film with a film thickness of 20 μmwas formed. The magnetic properties of the Ni-plated magnets are alsolisted in Table 4. Subsequently, after the Ni-plated surface wascleaned, the finally reachable degree of vacuum was measured under thesame conditions performed for the Example 4-1. The curve "m" indicatesthe results.

                  TABLE 4                                                         ______________________________________                                                      magnetic properties                                                           Br(kG)                                                                              iHc(kOe) (BH)max(MGOe)                                    ______________________________________                                        Example 4-1                                                                           this invension                                                                            11.0    16.0   30.0                                       Comparison                                                                            un-treated magnet                                                                         11.0    16.0   30.0                                       4-1                                                                           Comparison                                                                            Ni-platedmagnet                                                                           11.0    16.0   30.0                                       4-2                                                                           ______________________________________                                    

The R-Fe-B system permanent magnet, according to the present invention,having an external layer of Ti_(1-x) Al_(x) N coated layer formed on theAl coated layer which was previously formed onto the Ti coated layer hasdemonstrated that there was no gas generation, so that the finalreachable degree of vacuum of 1×10⁻⁹ Pa was achieved. On the other hand,with magnets without any further treatments or those being provided withthe Ni-plated layer, gas generation was found, causing the difficulty toreach the target degree of vacuum.

INDUSTRIAL APPLICABILITY

According to the present invention, by subsequent procedures of (1)cleaning the surface of R-Fe-B system permanent magnet by the surfacesputter method, (2) forming Ti coated film as a under coat by the thinfilm forming technique such as the ion plating method, and (3) formingeither TiN film layer, AlN film layer or Ti_(1-x) Al_(x) N as anexternal layer and/or Al layer or TIN_(x) layer as an intermediate layerby the ion reaction plating technique in N₂ -containing gas, the surfaceof the R-Fe-B system permanent magnet is coated with a dense andadherent film to prevent the gas generation, so that it is applicable tothe undulator used in the ultra-high vacuum atmosphere which saidundulator is required to exhibit excellent magnetic characteristics.

While this invention has been described with respect to preferredexamples, it should be understood that the invention is not limited tothat precise examples; rather many modifications and variations wouldpresent themselves to those of skill in the art without departing fromthe scope and spirit of this invention, as defined in the appendedclaims.

We claim:
 1. A magnet made of an R-Fe-B system alloy usable forultra-high vacuum with a layer consisting of Ti as an undercoat coatedon a surface layer of said magnet, either a TiN layer, an AlN layer or aTi_(1-x) Al_(x) N layer (where x: 0.03-0.70) as an external layer, andan Al layer inserted as an intermediate layer between the Ti undercoatlayer and the external layer.
 2. The magnet for ultra-high vacuumaccording to claim 1, wherein the Ti undercoat layer has a thickness of0.1 μm to 3.0 μm.
 3. The magnet for ultra-high vacuum according to claim1, wherein the the external layer consists of TiN and has a thickness of0.5 μm to 10 μm.
 4. The magnet for ultra-high vacuum according to claim1, wherein the external layer consists of AlN and has a thickness of 0.5μm to 10 μm.
 5. The magnet for ultra-high vacuum according to claim 1,wherein the external layer consists of Ti_(1-x) AlN (where x:0.03-0.70)and has a thickness of 0.5 μm to 10 μm.
 6. The magnet for ultra-highvacuum according to claim 1, wherein the Al intermediate layer has athickness of 0.1 μm to 5.0 μm.
 7. A production process for a magnetusable for ultra-high vacuum, comprising the sequential stepsof:cleaning a surface layer of an R-Fe-B system magnet whose main phaseconsists of a tetragonal phase; forming an undercoat layer consisting ofTi on the cleaned surface of valid R-Fe-B system magnet using a thinfilm forming method; forming an Al layer on the Ti undercoat layer usinga thin film forming method; and forming either one of a TiN layer, anAlN layer or a Ti_(1-x) Al_(x) N (where x is 0.03 to 0.70) layer as anexternal layer using a thin film forming method.
 8. The productionprocess for the magnet according to claim 7, wherein said thin filmforming method is either an ion plating or an evaporation method.
 9. Theproduction process for the magnet according to claim 7, wherein the theTi undercoat layer is formed to a thickness between 0.1 μm and 3.0 μm.10. The production process for the magnet according to claim 7, whereinthe external layer is formed of TIN to a thickness of 0.5 μm to 10 μm.11. The production process for the magnet according to claim 7, whereinthe external layer is formed of AlN to a thickness of 0.5 μm 10 μm. 12.The production process for the magnet according to claim 7, wherein thethe external layer is formed of Ti_(1-x) Al_(x) N (where x:0.03-0.70) toa thickness of 0.5 μm to 10 μm.
 13. The production process for themagnet according to claim 7, wherein the the intermediate Al layer isformed to a thickness of 0.1 μm to 5 μm.